Measuring the common emitter current gain β in a bipolar junction transistor

Dept. of Electrical, Computer and Biomedical Engineering

2 Measuring the common emitter current gain β in a BJT

Measuring β

β IC,VCE( )=IC

IB

B

IC

IE

IB

IE

IC

I

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Implementing a system allowing the user to measure the common emitter current gain β in NPN and PNP bipolar transistors. The system should include

Purpose of the experiment

The virtual instrument should represent β as a function of IC in graphical form and save the relevant data in the form of a table (IC, β) in a text file

a weighted resistor DAC controlling the collector current in the DUT (device under test)

a circuit setting the collector current of the transistor under test and VCB=0 (transistor in a diode connection)

a virtual instrument allowing the user to control the DAC, acquire the data relevant to the collector and base current in the DUT, compute β and represent it as a function of IC

Measuring the common emitter current gain β in a BJT

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The DAC use in this experience may be the same DAC implemented in one of the previous experiences

Ri should be chosen in such a way to have a maximum collector current of 10 mA; RB should be chosen so that the voltage Vo at the output of the second amplifier covers the input dynamic range of the data acquisition board on the PC (±10 V)

DUT

RB

Vo

Ri

B0

B7

DAC

Va CF

IC

IB

IC=-Va

Ri

, IB=- Vo

RB

β=Va

Vo

RB

Ri

CF=330 nF

Measuring the common emitter current gain β in a BJT

Measuring β

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TL081 JFET input OpAmp

Measuring the common emitter current gain β in a BJT

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TL081 JFET input OpAmp

Measuring the common emitter current gain β in a BJT

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Virtual Instrument

The only purpose of using this structure is that of introducing a clear time separation between the four tasks of the program:

•  DAC programming (frame #0)

•  acquisition of Va (frame #1)

•  acquisition of Vo (frame #2)

•  calculation and graphical representation of beta (frame #3)

Measuring the common emitter current gain β in a BJT

As far as the block diagram is concerned, the LabVIEW VI can be implemented by means of a sequence structure including 4 frames (to add a frame, right click on the frame of the structure and select “Add Frame After”). The sequence structure makes it possible to execute a set of instructions according to a user defined time sequence (first the instructions included in frame 0 are executed, then those included in frame 1, etc.)

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Measuring β as a function of IC

In order to measure the value of beta for different values of the collector current, the sequence structure can be included in a for loop with 256 iterations, one for each of the possible DAC output levels

Measuring the common emitter current gain β in a BJT

number of cycles

iteration number

9 Measuring the common emitter current gain β in a BJT

DAC programming (frame #0)

Measurement I/O -> DAQmx Data Acquisition -> DAQmx Write.vi

includes information on possible errors in one of the blocks in the chain

Measurement I/O -> DAQmx Data Acquisition -> DAQmx Create Channel.vi

specifies the name of the digital lines or the ID number of the ports used to create the virtual channel (Dev1/port0/line0:7)

Boolean control array

grouping of the digital lines in one or more virtual channels (one channel for each line)

DAQmx Create Channel.vi is used to configure the digital channel of the data acquisition board (DAQ, on the PC)

DAQmx Write.vi is used to set the value at the digital output channels configured by the previous function

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Acquiring Va e Vo (frame #1 and #2)

Measuring the common emitter current gain β in a BJT

DAQmx Create Channel.vi provides the acquisition board with information about the type and range of the signals to be acquired and about the input channel DAQmx Read.vi samples the signal from the specified channel and yield the measured value

input channel (Dev2/a1)

expected limits for the signal to be acquired

Measurement I/O -> DAQmx Data Acquisition -> DAQmx Create Channel.vi

Measurement I/O -> DAQmx Data Acquisition -> DAQmx Read.vi

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Prima di procedere con l’acquisizione di Vo è opportuno attendere che le correnti nel DUT raggiungano la condizione di regime. A questo scopo conviene inserire nel frame #0 un blocco di temporizzazione

Acquisition Timing

Waits until the timer content is a multiple of “millisecond multiple” before starting an iteration – generally used to synchronize the loop execution with the system clock

[ms multiple] 1 0 2 4 3 5

1st iteration 2nd iteration 3rd iteration

Waits for the specified number of milliseconds before starting an iteration 1st iteration

wait wait wait wait wait

2nd iteration 3rd iteration

Measuring the common emitter current gain β in a BJT

12 Measuring the common emitter current gain β in a BJT

XY graph (Modern-> Graph ->XY Graph o Classic ->Classic Graph ->XY Graph)

A Bundle function is required (Programming-> Cluster, Class & Variant-> Bundle) to group the independent (X) and dependent (Y) variables in a single array

Input data in the form of vectors (same size)

Suggestion: to generate the data vectors one could use tunnels in “indexed” modes in a for structure

Graphical representation of β(IC)

13 Measuring the common emitter current gain β in a BJT

Writing a file

Programming-> Array-> Build Array

input: 2 1D arrays

output: 1 2D array (2 column data table)

2D array transpose? yes

Programming-> File I/O-> Write To Spreadsheet File.vi

14 Measuring the common emitter current gain β in a BJT

FOR cycle for noise rejection

number of cycles

We can use a for cycle to reduce the effects of zero average disturbances, therefore improving the measurement accuracy

Instead of representing (in the graph or in the numeric indicator) each individual acquired sample of the signal, we can represent the average value of N samples – the speed at which the measurement result is represented on the graph will decrease by a factor of N

iteration number